UCS Power Question

[rchaseling]

Hi,

Just a quick one to double check I have this right....I have a UCS chassis with 2 x B200 servers & 2 x 2500W power supplies. Also will be running firmware 1.4.3m

1. When using only 2 PSUs do they have to be in particular slots? i.e 1&3

2. What would be the best power policy to use for separate power sources to the PSUs? Non-redunant?

3. What would be the best power policy to use for the same power source going  to both PSUs? N+1?

Also there will be more blades added in the future...is this going to make a difference to what I should choose?

Cheers

[chrisb2 · Accepted solution]

Hello!

The placement of the 2 PSUs do not matter as the system will adapt to where to you place them.

As for the power policy, it only dictates how the Chassis will use the PSUs it has:

Non-redunant - the chassis will use both PSUs to their full power, allowing you to use more blades

N+1 - One PSU will not be used and saved for a backup (thus in your configuration you could only use 1 PSU of blades)

Grid - Greater than N+1 Redundancy.

(source)

Because of this the choice of Power Policy is not related to what sources they are connected to, but how much redundancy you want.  Thus the safest choice would be N+1 (or Grid) with each PSU going to a different power source.  The most cost effective method is Non-redunant with all PSUs going to the same source, but if one of them goes down you lose blades (see below).

Please note that you need 3 PSUs to power a fully populated server. 

If you use Non-redunant Power Policy and one of the PSUs die, blades will be powered off until there is enough supply to satisfy the demand for power.  Thus it is much safer to use the N+1 setting.  If you add a new blade to the system and there is not enough power, you will simply get an error and the blade will not come up.

Does that answer your questions?

[rchaseling]

More than answers it. Cheers

[simon.geary]

The chassis can be powered by any two power supplies so it makes no difference what slots they go in.

Choosing quantity of power supplies and the power policy in UCS is very straightforward:

4 power supplies = Grid policy (use when you have two separate power feeds or require N+2 redundancy)

3 power supplies = N+1 policy

2 power supplies = non-redundant policy

So in your scenario it doesn't matter if the two power supplies are connected to separate power sources or not. You have two PSUs so your policy should be non-redundant.

And in a similar fashion, the number of blades in the chassis doesn't usually make any practical difference to the number of power supplies needed to run the chassis assuming you don't load it up so heavily that power requirements exceed 5000W which would be rare.

Only having two is obviously bad though as power is non-redundant. I would always recommend against only having two.

See this guide for more details:

http://www.cisco.com/en/US/prod/collateral/ps10265/ps10280/UCS_Power_Supply_Configuration_and_Provisioning.pdf

[rchaseling]

Guys,

So both answers are appreciated yet slightly different - I think my slight confusion is justified

Can I use N+1 with only 2 PSUs as Christopher mentions or do I have to use non redundant?

If I use non redundant and both PSUs are plugged into different sources and one goes down, providing that the chassis is not drawing more than 2500W would there be no disruption?

Cheers

[simon.geary · Accepted solution]

Hmm, now I am a little confused too!

The PDF link I posted is pretty clear, it says there must be at least three power supplies used with N+1.

However the chassis installation guide is also pretty clear, it says you can have two power supplies with N+1 assuming total draw is less than 2500W.

http://www.cisco.com/en/US/docs/unified_computing/ucs/hw/chassis/install/overview.html#wp1255314

In a hypothetical fight to the death between those two seemingly contradictory docs I think I would go with the chassis installation guide as being the more 'official' of the two, meaning you could indeed have two supplies in N+1.